As an image display device using light-emitting devices for pixels, an EL display using electroluminescence (hereinafter abbreviated to EL) elements is known. In an active matrix type EL display, wiring for conveying signals and electric currents is arranged in a matrix form, and each pixel has a built-in pixel circuit formed of a thin film transistor (hereinafter abbreviated to TFT), which is an active element, in addition to an EL element. The brightness of the EL element is controlled by regulating the current supplied to the EL element. A method for the pixel circuit to control the current is disclosed in, for instance, Patent Document 1. As an EL element whose brightness varies with the amperage, an organic EL diode is known.
FIG. 13 shows an example of configuration of a conventional image display device using EL elements. Over the surface of a glass substrate 91, an image display area 92 and a scanning circuit 94 are formed. In the image display area 92, a plurality of pixel circuits 95, a plurality of reset signal lines 96, a plurality of lighting signal lines 97, signal lines SL and power supply line PL are arranged in a matrix form. Each reset signal line 96 is connected to the reset signal inputs r of the pixel circuits 95 for one row, and each lighting signal lines 97, to the lighting signal inputs i of the pixel circuits 95 for one row. Each of the reset signal lines 96 and each of the lighting signal lines 97 serve to convey the output signals of the scanning circuit 94 to the pixel circuits 95 for one row. Each signal line SL is connected to the image signal inputs S of the pixel circuits 95 for one column, and each power supply line PL, to the power supply inputs P of the pixel circuits 95 for one column.
A driver IC 93 is bonded over the glass substrate 91 by pressure bonding. The driver IC 93 has a function to convert digital image signals serially received from outside into voltage signals and supply them to outputs D(1) through D(x). A power supply bus 98, connected to every one of the power supply lines PL, supplies a power voltage VDDex received from outside. The scanning circuit 94, which is a logic circuit formed of a TFT, has a function to drive every one of the reset signal lines 96 and the lighting signal lines 97.
The configuration of the pixel circuit 95 is the same as that of a pixel circuit 5 used in an embodiment of the present invention to be described later. As the detailed configuration and operation of the pixel circuit 5 will be described with reference to the embodiment, the operation of the pixel circuit 95 will not be described in detail here but only briefly.
Writing into a pixel circuit 95 causes the voltage of sum (Vdata+Vth) of a signal voltage Vdata and the absolute value Vth of the threshold voltage of a TFT 21 to be stored into a capacitor 24. When an image is to be displayed, the image signal inputs S to the pixel circuits are kept constant and a TFT 23 is turned on. Then, the voltage (Vdata+Vth) is generated between the gate and source of the TFT 21, to cause a current to flow into an EL element 25. As the amperage of the current flowing into the EL element 25 is controlled with the image signal voltage Vdata, the pixel circuit 95 can control the brightness of the EL element 25. By varying the image signal voltage Vdata to be written into each pixel circuit 95 according to the image, the intended image can be displayed.
Patent Document 1: Japanese Patent Laid-Open No. 2003-122301